Fig. 16.1 illustrates action potentials found in different cardiac cells. Two main types of action potentials occur in the heart and are depicted. One type, the fast response, occurs in normal atrial and ventricular myocytes and in the specialized conducting fibers (Purkinje fibers of the heart) and is divided into five phases. The rapid upstroke of the action potential is designated phase 0. The upstroke is followed by a brief period of partial, early repolarization (phase 1) and then by a plateau (phase 2) that persists for approximately 0.1 to 0.2 second. The membrane then repolarizes (phase 3) until the resting state of polarization (phase 4) is again attained. Final repolarization (phase 3) develops more slowly than depolarization (phase 0). The other type of action potential, the slow response, occurs in the sinoatrial (SA) node, which is the natural pacemaker region of the heart, and in the atrioventricular (AV) node, which is the specialized tissue that conducts the cardiac impulse from the atria to the ventricles. The slow-response cells lack the early repolarization phase (phase 1).
Other differences between the electrical properties of the fast-response and slow-response cells include the following: The resting membrane potential (phase 4) of the fast-response cells is considerably more negative than that of the slow-response cells. Moreover, the slope of the upstroke (phase 0), the amplitude of the action potential, and the overshoot (membrane voltage positive to 0 mV) are greater in the fast-response cells than in the slow-response cells. The action potential amplitude and the steepness of the upstroke are important determinants of propagation velocity along the myocardial fibers. In slow-response cardiac tissue, the action potential is propagated more slowly and conduction is more likely to be blocked than in fast-response cardiac tissue. Slow conduction and a tendency toward conduction block increase the likelihood of some rhythm disturbances (see the section “
The action potential initiates contraction of the myocyte. The relationship between the action potential and contraction of a cardiac myocyte is shown in
Fig. 16.2 . Rapid depolarization (phase 0) precedes cell shortening, and completion of repolarization occurs just before peak shortening. Relaxation of the muscle takes place mainly during phase 4 of the action potential. The duration of contraction usually parallels the duration of the action potential.
The various phases of the cardiac action potential are associated with changes in cell membrane permeability, mainly by Na+, K+, and Ca++ ions. Changes in cell membrane permeability alter the rate of movement of these ions across the membrane and thereby change the membrane voltage (Vm). These changes in permeability are accomplished by the opening and closing of ion channels that are specific for individual ions (see
As with all other cells in the body, the concentration of K+ inside a cardiac muscle cell (intracellular [K+]) exceeds the concentration outside the cell (extracellular [K+]). The concentration gradient is reversed for Na+ and Ca++ . Estimates of the extracellular and intracellular concentrations of Na+, K+, and Ca++ and the Nernst equilibrium potentials (see
Eq. 1.5b ) for these ions are compiled in
Table 16.1
The resting cell membrane has a relatively high permeability for K+; permeability for Na+ and Ca++ is much less. Because of the existing chemical gradient for K+ and Vm, K+ tends to diffuse from the inside to the outside of the cell.
3 Any diffusion of K+ that occurs at the resting membrane potential (i.e., during phase 4) takes place mainly through 4 specific potassium channels. Several types of potassium channels exist in cardiac cell membranes. Opening and –120
ERP RRP closing of some of these channels are regulated by Vm, whereas others are controlled by a chemical signal (e.g., the extracellular acetylcholine concentration). The specific 4 is a voltage-regulated channel that conducts the inward rectifying K+ current (iK1), which is discussed in more detail later. For now, it is necessary only to know how this current is established.